1. Field of the Invention
The present invention relates to a control unit with circuit board comprising at least one track and a transistor mounted on the circuit board and electrically connected with at least one track.
2. Description of the Related Art
These types of control units, which comprise a transistor as semiconductor control element, are used in a large number of applications, for example also for the control of drive, heating and lighting devices in motor vehicles and particularly where, due to the many devices to be controlled in an overall apparatus, it is important to manufacture the individual control units as economically as possible.
Various applications are conceivable in which, for example, due to the current to be controlled and passed through the track, overheating of the control unit may occur. Such malfunctions are in particular caused by previous damage to the transistor. During the mass production of transistors, this previous damage only occurs in the parts per million (ppm) range. However, since the transistors, in particular with inexpensive mass production, are arranged on injection moulded parts or are surrounded by them, overheating of the transistors also leads to overheating of the surrounding plastic parts. The plastic begins to smoulder at the temperatures of overheating, for example in the region of 300° C., and in the worst case it bums. Consequently, the device in which the control unit is installed can be partially or completely destroyed.
Such control units are also used, for example, in electrical heating devices for motor vehicles, such as is known, for example, from EP-A-0 359 528. For applications in motor vehicles, in particular in motor vehicles with consumption-optimised diesel engines and, in future, also petrol-driven engines in which a smaller amount of the heat energy arises, electrical auxiliary heaters are used for heating the interior and engine. Electrical heating devices are also suitable for other applications, for example in the field of house installations, in particular for room air conditioning, industrial plants, etc.
Through the use of electrical auxiliary heaters in the heating/air conditioning system of a motor vehicle, the time period can be spanned in which the waste heat from the engine is not yet available for heating purposes. Electrical auxiliary heaters reach their operating temperature after just a few seconds and can therefore accordingly heat up the air flowing through.
For such electrical auxiliary heaters, which are employed in heating/air conditioning systems in motor vehicles, Positive Temperature Coefficient (PTC) heating elements are preferably used which convert the electrical current into heat. The PTC heating elements are joined to radiator elements of the auxiliary heating for thermal conduction. The heat produced by the PTC heating elements is transferred to the air flowing through by means of the radiator elements.
The overall arrangement of a layered structure of PTC heating elements, radiator elements and contact plates, which provide the current feed, is subjected to a clamping pressure to increase the efficiency. Due to the clamping, an improvement in the electrical and thermal contact to the PTC heating elements is achieved.
The control of the heating power is being handled to an increasing extent by semiconductor components which replace relay controllers. In dependence of the demanded heating power, the control units control in a closed-loop mode the current fed to the individual PTC heating elements. Preferably, the semiconductor components operate as switches which switch the current passed to the individual PTC heating elements on or off. For more exact dosing of the heating power the current passed to the heating elements can with the aid of the semiconductor components also be continuously adjusted. A disadvantage of such a continuous closed-loop control is the ensuing high power dissipation which is converted into heat and can therefore also contribute to overheating.
The semiconductor components used for regulating the current can be obtained in a large number of variants with different additional functions. Complex and expensive transistors have many of so-called “smart power control” functions. Using them, it is possible, for example, to realise automatic temperature monitoring of the transistor. A disadvantage with such transistors is however their high price, so that they are only used in high priced vehicles.
With the overheating of this type of control device of an auxiliary heater in a motor vehicle, the gases produced by the smouldering plastics spread directly through the ventilation system of the motor vehicle and enter the passenger compartment. Then persons located in the passenger compartment directly suffer health impairment due to the emitted gases. Also the emitted gases cloud up the passenger compartment and impair the view of the driver towards the outside and even completely block it.
Even with temperature monitoring of the transistor or through different electronic monitoring of the actual temperature for the timely switching off of the currents before overheating occurs, there is also the problem of insufficient reliability of the overheating protection which is thereby implemented. Also electrical overheating protection of the control unit represents an unsatisfactory solution, because also here the required reliability is not always achieved. Anyway, with high currents to be monitored, the realization of electrical overheating protection leads to a control unit of substantial installation size.